There are a few ways to block a specific device from accessing your Wi-Fi network, depending on your specific network setup and equipment. Here are a few options:

1. Change your Wi-Fi password: The most straightforward way to block a device from accessing your Wi-Fi network is to change the Wi-Fi password. This will prevent the device from being able to connect to your network without the new password.

2. Use MAC address filtering: Every device has a unique MAC address that can be used to restrict access to your network. Many routers have a feature that allows you to create a list of allowed devices based on their MAC addresses. You can set up your router to only allow specific devices to connect to your network and block any others.

3. Enable guest network: Some routers have a guest network feature that you can use to provide internet access to visitors without giving them access to your main network. You can create a separate guest network and only allow specific devices to connect to it, effectively blocking any devices you don't want to have access.

4. Use parental controls: If your router has parental control features, you may be able to use them to block specific devices from accessing your network. You can set up access schedules or content filters to restrict access to certain devices.

It's important to note that none of these methods are foolproof and determined users may still find ways to access your network. However, using a combination of these methods can help make it more difficult for unauthorized devices to connect to your network.